1.1 Field of the Invention
The invention relates generally to interpretation of images and specifically to determination of the orientation of lines of text.
1.2 Description of the Prior Art
An important area of image interpretation is optical character recognition, in which images of symbols are automatically translated into binary codes representing the symbols. A major problem in optical character recognition is determining the proper orientation of the symbols. The problem can occur in three contexts: first, the user of the optical character recognition apparatus (OCR) can fail to properly align a page of text as he places it on the platen of the imaging component of the OCR. Second, the OCR, may be employed to deal with texts written with their lines in different orientations. For instance, a page of English may be in landscape style, with the lines running in the long direction of the page, or may even have lines written vertically. Some writing systems require vertical lines; in others the lines may be written vertically or horizontally. Finally, a single page may have components in which the lines are oriented in different directions. In all of these situations, the OCR system cannot begin interpreting the symbols of the text until it has determined the orientation of the lines which contain them.
Prior techniques for determining the orientation of lines have fallen into two classes:
1. those which begin with a prior determination of the nominal orientation of the lines and attempt to correct only for alignment errors made by the users and PA1 2. those which determine the orientation of the lines without prior assumptions.
An example of the first type of technique may be found in U.S. Pat. No. 5,001,766, H. S. Baird, Apparatus and Method for Skew Control of Document Images, issued Mar. 19, 1991. Examples of the second type of technique are found in L. O'Gorman, "The Document Spectrum for Page Layout Analysis," in: Int'l Association for Pattern Recognition Workshop on Structural and Syntactic Pattern Recognition, 1992 and in Akihide Hashizume, et al., "A method of detecting the orientation of aligned components", in: Pattern Recognition Letters, April 1986.
The first type of technique requires human intervention if the OCR system is dealing with documents having more than one kind of line orientation; prior-art versions of the second type of technique have not worked well in difficult cases. One such difficult case has been documents written with character sets having multiple connected components. Examples of such character sets include Chinese ideograms, the Korean Hangul alphabetic script, or heavily-accented writing systems such as Thai. These problems are overcome by the techniques described below. The techniques permit OCR, systems to automatically handle pages having blocks of text with different orientations. Moreover, the techniques work well with character sets having multiple connected components.